From 089687fbcc6524809ae9f4b2f8145fe3c2a91147 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pali=20Roh=C3=A1r?= <pali@kernel.org>
Date: Sat, 7 Aug 2021 19:48:01 +0200
Subject: [PATCH] pppd: Retry registering interface when on rtnetlink -EBUSY
 error
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Due to workaround in kernel module ppp_generic.ko in function
ppp_nl_newlink(), kernel may return -EBUSY error to prevent possible
mutex deadlock. In this case userspace needs to retry its request.

Proper way would be to fix kernel module to order requests and mutex
locking, so prevent deadlock in kernel and so never return this error to
userspace. Until it happens we need retry code in userspace.

Signed-off-by: Pali Rohár <pali@kernel.org>
[ backport to ppp 2.4.9 ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
---
 pppd/sys-linux.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/pppd/sys-linux.c
+++ b/pppd/sys-linux.c
@@ -707,99 +707,101 @@ static int make_ppp_unit_rtnetlink(void)
     int one;
     int fd;
 
-    fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
-    if (fd < 0) {
-        error("make_ppp_unit_rtnetlink: socket(NETLINK_ROUTE): %m (line %d)", __LINE__);
-        return 0;
-    }
-
-    /* Tell kernel to not send to us payload of acknowledgment error message. */
-    one = 1;
-    setsockopt(fd, SOL_NETLINK, NETLINK_CAP_ACK, &one, sizeof(one));
+    do {
+        fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
+        if (fd < 0) {
+            error("make_ppp_unit_rtnetlink: socket(NETLINK_ROUTE): %m (line %d)", __LINE__);
+            return 0;
+        }
+
+        /* Tell kernel to not send to us payload of acknowledgment error message. */
+        one = 1;
+        setsockopt(fd, SOL_NETLINK, NETLINK_CAP_ACK, &one, sizeof(one));
+
+        memset(&nladdr, 0, sizeof(nladdr));
+        nladdr.nl_family = AF_NETLINK;
+
+        if (bind(fd, (struct sockaddr *)&nladdr, sizeof(nladdr)) < 0) {
+            error("make_ppp_unit_rtnetlink: bind(AF_NETLINK): %m (line %d)", __LINE__);
+            close(fd);
+            return 0;
+        }
+
+        memset(&nlreq, 0, sizeof(nlreq));
+        nlreq.nlh.nlmsg_len = sizeof(nlreq);
+        nlreq.nlh.nlmsg_type = RTM_NEWLINK;
+        nlreq.nlh.nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK | NLM_F_EXCL | NLM_F_CREATE;
+        nlreq.ifm.ifi_family = AF_UNSPEC;
+        nlreq.ifm.ifi_type = ARPHRD_NETROM;
+        nlreq.ifn.rta.rta_len = sizeof(nlreq.ifn);
+        nlreq.ifn.rta.rta_type = IFLA_IFNAME;
+        strlcpy(nlreq.ifn.ifname, req_ifname, sizeof(nlreq.ifn.ifname));
+        nlreq.ifli.rta.rta_len = sizeof(nlreq.ifli);
+        nlreq.ifli.rta.rta_type = IFLA_LINKINFO;
+        nlreq.ifli.ifik.rta.rta_len = sizeof(nlreq.ifli.ifik);
+        nlreq.ifli.ifik.rta.rta_type = IFLA_INFO_KIND;
+        strcpy(nlreq.ifli.ifik.ifkind, "ppp");
+        nlreq.ifli.ifid.rta.rta_len = sizeof(nlreq.ifli.ifid);
+        nlreq.ifli.ifid.rta.rta_type = IFLA_INFO_DATA;
+        nlreq.ifli.ifid.ifdata[0].rta.rta_len = sizeof(nlreq.ifli.ifid.ifdata[0]);
+        nlreq.ifli.ifid.ifdata[0].rta.rta_type = IFLA_PPP_DEV_FD;
+        nlreq.ifli.ifid.ifdata[0].ppp.ppp_dev_fd = ppp_dev_fd;
+
+        memset(&nladdr, 0, sizeof(nladdr));
+        nladdr.nl_family = AF_NETLINK;
+
+        memset(&iov, 0, sizeof(iov));
+        iov.iov_base = &nlreq;
+        iov.iov_len = sizeof(nlreq);
+
+        memset(&msg, 0, sizeof(msg));
+        msg.msg_name = &nladdr;
+        msg.msg_namelen = sizeof(nladdr);
+        msg.msg_iov = &iov;
+        msg.msg_iovlen = 1;
+
+        if (sendmsg(fd, &msg, 0) < 0) {
+            error("make_ppp_unit_rtnetlink: sendmsg(RTM_NEWLINK/NLM_F_CREATE): %m (line %d)", __LINE__);
+            close(fd);
+            return 0;
+        }
+
+        memset(&iov, 0, sizeof(iov));
+        iov.iov_base = &nlresp;
+        iov.iov_len = sizeof(nlresp);
+
+        memset(&msg, 0, sizeof(msg));
+        msg.msg_name = &nladdr;
+        msg.msg_namelen = sizeof(nladdr);
+        msg.msg_iov = &iov;
+        msg.msg_iovlen = 1;
+
+        nlresplen = recvmsg(fd, &msg, 0);
+
+        if (nlresplen < 0) {
+            error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): %m (line %d)", __LINE__);
+            close(fd);
+            return 0;
+        }
 
-    memset(&nladdr, 0, sizeof(nladdr));
-    nladdr.nl_family = AF_NETLINK;
-
-    if (bind(fd, (struct sockaddr *)&nladdr, sizeof(nladdr)) < 0) {
-        error("make_ppp_unit_rtnetlink: bind(AF_NETLINK): %m (line %d)", __LINE__);
         close(fd);
-        return 0;
-    }
-
-    memset(&nlreq, 0, sizeof(nlreq));
-    nlreq.nlh.nlmsg_len = sizeof(nlreq);
-    nlreq.nlh.nlmsg_type = RTM_NEWLINK;
-    nlreq.nlh.nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK | NLM_F_EXCL | NLM_F_CREATE;
-    nlreq.ifm.ifi_family = AF_UNSPEC;
-    nlreq.ifm.ifi_type = ARPHRD_NETROM;
-    nlreq.ifn.rta.rta_len = sizeof(nlreq.ifn);
-    nlreq.ifn.rta.rta_type = IFLA_IFNAME;
-    strlcpy(nlreq.ifn.ifname, req_ifname, sizeof(nlreq.ifn.ifname));
-    nlreq.ifli.rta.rta_len = sizeof(nlreq.ifli);
-    nlreq.ifli.rta.rta_type = IFLA_LINKINFO;
-    nlreq.ifli.ifik.rta.rta_len = sizeof(nlreq.ifli.ifik);
-    nlreq.ifli.ifik.rta.rta_type = IFLA_INFO_KIND;
-    strcpy(nlreq.ifli.ifik.ifkind, "ppp");
-    nlreq.ifli.ifid.rta.rta_len = sizeof(nlreq.ifli.ifid);
-    nlreq.ifli.ifid.rta.rta_type = IFLA_INFO_DATA;
-    nlreq.ifli.ifid.ifdata[0].rta.rta_len = sizeof(nlreq.ifli.ifid.ifdata[0]);
-    nlreq.ifli.ifid.ifdata[0].rta.rta_type = IFLA_PPP_DEV_FD;
-    nlreq.ifli.ifid.ifdata[0].ppp.ppp_dev_fd = ppp_dev_fd;
-
-    memset(&nladdr, 0, sizeof(nladdr));
-    nladdr.nl_family = AF_NETLINK;
-
-    memset(&iov, 0, sizeof(iov));
-    iov.iov_base = &nlreq;
-    iov.iov_len = sizeof(nlreq);
-
-    memset(&msg, 0, sizeof(msg));
-    msg.msg_name = &nladdr;
-    msg.msg_namelen = sizeof(nladdr);
-    msg.msg_iov = &iov;
-    msg.msg_iovlen = 1;
-
-    if (sendmsg(fd, &msg, 0) < 0) {
-        error("make_ppp_unit_rtnetlink: sendmsg(RTM_NEWLINK/NLM_F_CREATE): %m (line %d)", __LINE__);
-        close(fd);
-        return 0;
-    }
-
-    memset(&iov, 0, sizeof(iov));
-    iov.iov_base = &nlresp;
-    iov.iov_len = sizeof(nlresp);
-
-    memset(&msg, 0, sizeof(msg));
-    msg.msg_name = &nladdr;
-    msg.msg_namelen = sizeof(nladdr);
-    msg.msg_iov = &iov;
-    msg.msg_iovlen = 1;
-
-    nlresplen = recvmsg(fd, &msg, 0);
-
-    if (nlresplen < 0) {
-        error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): %m (line %d)", __LINE__);
-        close(fd);
-        return 0;
-    }
-
-    close(fd);
 
-    if (nladdr.nl_family != AF_NETLINK) {
-        error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): Not a netlink packet (line %d)", __LINE__);
-        return 0;
-    }
-
-    if ((size_t)nlresplen < sizeof(nlresp) || nlresp.nlh.nlmsg_len < sizeof(nlresp)) {
-        error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): Acknowledgment netlink packet too short (line %d)", __LINE__);
-        return 0;
-    }
-
-    /* acknowledgment packet for NLM_F_ACK is NLMSG_ERROR */
-    if (nlresp.nlh.nlmsg_type != NLMSG_ERROR) {
-        error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): Not an acknowledgment netlink packet (line %d)", __LINE__);
-        return 0;
-    }
+        if (nladdr.nl_family != AF_NETLINK) {
+            error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): Not a netlink packet (line %d)", __LINE__);
+            return 0;
+        }
+
+        if ((size_t)nlresplen < sizeof(nlresp) || nlresp.nlh.nlmsg_len < sizeof(nlresp)) {
+            error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): Acknowledgment netlink packet too short (line %d)", __LINE__);
+            return 0;
+        }
+
+        /* acknowledgment packet for NLM_F_ACK is NLMSG_ERROR */
+        if (nlresp.nlh.nlmsg_type != NLMSG_ERROR) {
+            error("make_ppp_unit_rtnetlink: recvmsg(NLM_F_ACK): Not an acknowledgment netlink packet (line %d)", __LINE__);
+            return 0;
+        }
+    } while (nlresp.nlerr.error == -EBUSY);
 
     /* error == 0 indicates success, negative value is errno code */
     if (nlresp.nlerr.error != 0) {
